A conventional window assembly comprises a window or sash surrounded by a mounting frame which fits in a wall opening. The frame comprises opposite side frame jambs and the sash has stiles which fit with the frame jambs. The jambs provide a surrounding jig or guide for the sash stiles. When a worker handles a conventional window unit, he or she may pick up by the jamb which causes an outward bowing of the jamb relative to the sash stile. This can result in the sash falling out of the frame. Although there are currently some different types of hardware fitted for example with the balance shoes which are specifically designed to provide a localized interlock between the hardware mounted on the sash and the frame, there is nothing currently available in the way of a sash stile to frame jamb interlock which would help to eliminate outward bowing of the frame relative to the sash.
Even when the frame is fitted into the window opening, the window opening itself is likely to be oversized relative to the frame to ensure that the frame can fit into the opening. A good installer must then set shims behind the jamb or use jamb adjustment clips to ensure that the jamb remains straight relative to the sash. This can be time consuming and therefore a labour expensive installation.